1. Field of the Invention
The present invention relates to a spectacle lens used to assist an accommodation power of an eye and, more particularly, to a progressive power lens, a refracting power of which is continuously changed in its intermediate portion.
2. Related Background Art
As spectacle lenses for assisting an accommodation power when an accommodation power of an eye becomes too weak to see a near point, various progressive power lenses have been known. A progressive power lens of this type has an upper hyperopia correcting region (to be referred to as a "farsight portion" hereinafter), a lower myopia correcting region (to be referred to as a "nearest portion" hereinafter), and a progressive region (to be referred to as an "intermediate portion" hereinafter) which is located between the above two portions and in which a refracting power is continuously changed.
In a progressive power lens of this type, in general, when wide distinct vision areas of the farsight and nearsight portions are kept and are coupled by a progressive power range in which a refracting power is continuously changed, a lens aberration is concentrated on a side region of the progressive power range. Thus, the presence of this region leads to blurring and distortion of an image, and a user who wears spectacles experiences a fluctuation of an image when he or she turns his or her eyes.
In order to prevent such a problem of visual characteristics, in known progressive power lenses, designs and evaluations have been made from a variety of viewpoints. As for the shape of a lens surface, a crossing curve defined by a section extending along a vertical meridian vertically passing through substantially the center of a lens surface and an object-side lens surface is used as a reference curve for expressing specifications such as a dioptric power of a lens, and is also used as an important reference curve in a lens design. In a progressive power lens in which a nearsight portion is asymmetrically arranged in consideration of the fact that the nearsight portion comes close to a nose side when the lens is worn, a vertical central line passing through the centers of both the farsight and nearsight portions is used as a reference line. In the present invention, these reference curve and line will be referred to as "a principal meridian curve" hereinafter.
As conventional progressive power lenses, lenses disclosed in, e.g., U.S. Pat. Nos. 3,687,528 and 3,910,691, Japanese Patent Publication No. 59-42285, and the like are known.
These prior arts can improve visual characteristics to some extent. However, such improvements are insufficient in practical applications. More specifically, in a lens disclosed in U.S. Pat. No. 3,687,528, as for the shapes of crossing curves defined by a plane perpendicular to the principal meridional curve and a refracting surface of a lens, only a crossing curve at a point corresponding to substantially the center of an intermediate portion defines a circular arc. Crossing curves above and below the point define non-circular arcs in which a radius of curvature is decreased as the crossing curve is spaced apart from the principal meridional curve in a portion above the point, and vice versa in the a portion below the point. Therefore, in the lens disclosed in U.S. Pat. No. 3,687,528, only the center of the intermediate portion defines a circular arc, and other portions define simple non-circular arcs. Therefore, distinct vision areas (ranges with an astigmatic difference of 0.5 D (diopter) or less) of the farsight and nearsight portions become narrow, and a field of view is also narrowed due to an abrupt change in aberration, resulting in considerable distortion and fluctuation of an image.
In a lens disclosed in U.S. Pat. No. 3,910,691, the dynamic vision such as fluctuation of an image is improved in addition to an improvement in the static vision in the lens disclosed in U.S. Pat. No. 3,687,528. However, although the dynamic vision can be improved to some extent, it is still difficult to attain a sufficient performance in practical use.
In a lens disclosed in Japanese Patent Publication No. 59-42285, as for the shapes of crossing lines each defined by a plane perpendicular to the principal meridional curve and a refracting surface of a lens, the crossing lines define non-circular arcs in an upper portion of the farsight portion. More specifically, in the upper portion of the farsight portion, the radius of curvature of each crossing curve is decreased as it goes away from the principal meridional curve, a rate of decrease in radius of curvature approaches 0 toward an upper peripheral portion, and a crossing curve near the upper peripheral portion has a constant radius of curvature. In a lower portion of the farsight portion, the radius of curvature of each crossing curve is linearly decreased. In the intermediate portion, crossing curves define non-circular arcs except for a connecting portion with the farsight portion, in which the radius of curvature of each crossing curve is increased and then decreased as it goes away from the principal meridional curve. In the nearsight portion, the radius of curvature is increased and then decreased as it goes away from the principal meridional curve. In this lens, visual characteristics can be improved to some extent as compared to those disclosed in U.S. Pat. Nos. 3,910,691 and 3,687,528. However, in a peripheral region of the farsight portion, in particular, in a side region between the central and lower portions of the farsight portion, a residual astigmatic difference is still considerable. In addition, side regions of the intermediate and farsight portions still suffer from large distortion and fluctuation of an image. Thus, it is yet difficult to obtain a wide field of view.
In general, when the intermediate portion as a progressive power range extending along the principal meridional curve is shortened, an aberration is immediately increased since a refracting power undergoes an abrupt change. As can be seen from the Minkwittz's law, especially near the principal meridional curve, an aberration tends to be abruptly increased and the width of the progressive power range tends to be decreased. In addition, fluctuation and distortion of an image are abruptly increased. On the other hand, when the progressive power range is relatively long, an astigmatic difference, and fluctuation and distortion of an image can be easily eliminated since a change in refracting power is relatively slow. However, if the progressive power range is too long, a desired dioptric power cannot be obtained unless a user sufficiently lowers his or her eye point when he or she wears such spectacles, resulting in inconvenience.